Interest in crystallization, and in various ways for altering the shapes and structures of crystals, has a long history. Efforts to modify crystallization processes so as to generate new crystalline forms of substances continue to be of considerable importance for various reasons including, for example, improvement of mass-handling characteristics of particulate materials, production of materials that are stronger or more durable than existing materials, and production of materials having improved physical characteristics such as light transmissivity.
One traditional way in which alteration of the shape (i.e., the "habit") or the crystal lattice (i.e., the "morphology") of a crystalline material has been achieved is by the use of additives. Weissbuch et al., Science 253:637-645 (1991); Addadi et al., Topics in Stereochem. 16:1-85 (1986); Addadi et al., Angew. Chem. Int. Ed. Engl. 24:466-485 (1985); and Addadi et al., Nature 296:21-26 (1982). Additive effects can be dramatic. A notorious example is the conversion of cubic NaCl to octahedral NaCl in the presence of traces of urea. Often, however, additives can incorporate into the crystals during growth, which is undesirable especially if a highly pure crystalline substance is needed.
Another traditional way to alter the habit or morphology of a crystalline material is by changing the crystallization solvent (including crystallization from the gas phase) used to dissolve the crystallization solute. Unfortunately, changing the crystallization solvent may not be possible because the crystallization solute must be soluble in the solvent employed and few alternative solvents may be available. For example, very polar solutes such as salts are typically crystallized from very polar solvents, especially water, because such solutes are insoluble in less polar solvents. Similarly, formation of materials for many electronic or optical uses (e.g., metal chalcogenides) via precipitation upon admixture of precursor salt solutions also requires very polar media in order to dissolve the precursor solutes. Although various ways of avoiding the requirement of aqueous solvents for the formation of such materials have appeared, these techniques often require the preparation of organoelement precursors or use of toxic and volatile inorganics and are far from being generally applicable.
Salts such as potassium chloride, which are freely soluble in water, are insoluble in non-polar solvents. Solvent effects on the alteration of crystal growth of such salts have not been explored to any great extent simply because one must dissolve the salt before it can be crystallized from solution.
Complexation using crown ethers provides a convenient method of solubilizing salts, in the form of their complexes, in non-polar solvents, as first reported by Pedersen, J. Am. Chem. Soc. 89.:2495-2496 (1967); see also, Pedersen, J. Am. Chem. Soc. 89:7017-7036 (1967). Doxsee et al., J. Inclus. Phenom. & Molec. Recog. in Chem. 9:327-336 (1990) reports the crystallization of a simple organic salt, sodium acetate, from cyclohexane solution in which sodium acetate is completely insoluble. The sodium acetate solute is solubilized as a crown-ether complex of the salt in the organic solvent. Slow evaporation of the solution results in precipitation of the free salt in crystalline form. When sodium acetate is conventionally crystallized from an aqueous solution, the crystals have a hexagonal plate habit. Crystallization of sodium acetate from the organic solution of its crown ether complex results in the formation of needle-shaped crystals. Despite their quite different crystal habits, the needles have an identical crystal lattice as the hexagonal crystals, as ascertained by x-ray diffraction analysis.
Thus, addition of complexation agents, such as crown ethers, solubilizes compounds in solvents in which the compounds would normally be completely insoluble. Initiation of crystallization then provides an opportunity to observe crystal growth under solvation conditions that are quite different from those of "normal" crystal-growth conditions.
Whereas crystallization of a simple inorganic salt from organic solution is intriguing, the method has substantial limitations. For example, it is limited to production of crystals of the particular solute already dissolved in the solution. Production of a large variety of other crystalline materials requires chemical reactions between reactants supplied by more than one solute.
Production of many commercially important compounds using conventional reaction technology is often difficult, hazardous, and/or expensive. Suitable reaction precursors (i.e., "reactants") may be unavailable or very expensive; the reactants may be highly toxic or flammable or have some other undesirable property such as extreme volatility. Finally, certain reactants are inefficient and generate undesired by-products or require additional process steps to obtain the desired product at a suitable purity level. For example, zinc selenide is conventionally produced by reacting dimethyl zinc with hydrogen selenide. Dimethyl zinc is expensive, extremely volatile, and pyrophoric. It would be advantageous if zinc selenide could be produced from an inorganic salt such as zinc chloride that is inexpensive and safe to use.
Therefore, there is a need for methods for producing, via a chemical reaction, materials having crystalline forms that are different in habit from crystalline materials produced by simple crystallization from solution. Materials having such different crystalline forms can be more suitable for particular applications.
There is also a need for such method permitting production of such crystalline forms using reactants that are less hazardous to work with compared to reactants that must be used in conventional reaction systems.